The present invention relates to an automatic slack adjuster for cam-actuated brakes of a motor vehicle, and more particularly to a mechanism for automatically taking up the slack which develops between the movable elements of a vehicle brake and the member which transmits the operating force thereto as the brake lining and the mechanical parts cooperating therewith undergo normal wear.
Known types of slack adjusters for cam-actuated brakes of vehicles, especially of the heavy-duty variety, generally have utilized a manual setting arrangement for each adjuster, making it necessary to check and readjust each brake shoe slack adjuster periodically. To do this, brakes are manually snugged up to create a heavy drag and then loosened for a half or more turns of an adjusting screw. The adjustment is then set where the mechanic thinks he has a free wheel, thereby providing a fairly short brake chamber and push rod travel. As the brakes are repeatedly applied, lining wear occurs, thus gradually increasing push rod travel to the point where readjustment is required. The vehicle must, therefore, be returned to the shop for manual adjustment of the brakes at short intervals. One major disadvantage of manual brake adjustment is the likelihood, on the part of the mechanic, of making adjustments on the tight side rather than on the loose side to avoid the usual kickback in a short time that the brakes are still too loose. Obviously, the tight adjustment increases the wear on the linings, provides excessive drag and heat, and creates a much shorter brake life.
To overcome the disadvantages of manually set slack adjusters, more recent slack adjusters have the feature of taking up the wear on each individual wheel by automatic means. However, such presently known adjusters for cam-actuated brake systems are subject to certain limitations. Most known adjusters provide an adjustment of the cam shaft and cam in response to the total angular rotation of the cam and shaft. A portion of this cam rotation is utilized to move the brake shoes from the non-braking position to a braking position where the brake shoes contact the drum; and a further portion of cam rotation occurs after initial contact and results from distortion or deformation of the brake drum, brake lining, brake shoes, cam shaft and air chamber bracket.
Most previously known automatic adjusters have been unable to distinguish between the brake shoe movement over the normal running clearance to initial braking contact with the drum and addition rotation of the cam due to the distortion or deformation above described. Also, because of such deformation, prior known adjusters will provide adjustment as a consequence of a single very hard application of the brakes, although no adjustment is really required; so that as a result, the slack for running clearance will become too small. The danger of such false adjustment is particularly great where a hand brake is connected to a power operated brake system. The present invention overcomes these above recited limitations to provide an improved automatic slack adjuster system.
Among the objects of the present invention is the provision of a novel automatic slack adjuster for vehicle brakes which compensates for brake lining wear and utilizes a torque limiting one-way spring clutch acting to adjust the slack for a cam-actuated brake during brake application. A linkage mechanism between an actuating rod of a braking power source and the shaft for a worm gear of the adjustment assembly is directly connected to a driving member of the torque limiting clutch for the worm shaft, and a friction member prevents unintentional worm shaft rotation when the brake shoes are not engaged with the drum.
Another object of the present invention is the provision of a novel automatic slack adjuster for vehicle brakes wherein the combination of an actuator, one-way spring clutch and a torsion spring together with an adjustable worm and worm gear combination is sealed within the operating arm member. The worm shaft, actuator, one-way spring clutch and torsion spring are all mounted in the member in concentric relationship. During brake application, assuming some lining wear has occurred, the worm and worm gear are under load after about one-half the full stroke has occurred; at which point rotation of the actuator occurs while the brake shoes have moved to firmly engage the drum. As the worm is under load and cannot rotate, the one-way spring clutch overrides the worm shaft. Upon release of the brake, the torsion spring tends to rotate the actuator in the opposite direction, but cannot because the one-way clutch is now drivingly connected to the worm shaft which is still under load. As the brake shoes disengage from the drum, the load on the worm gear and the worm shaft is reduced to a point where the torsion spring can now return the actuator, thereby advancing the worm on the gear and moving the operating arm in a slack reducing direction. A friction member prevents unintentional rotation of the worm shaft when the shaft is not otherwise loaded.
A further object of the present invention is the provision of a novel automatic slack adjuster for brakes wherein a limit stop is provided in the operating arm member cooperating with the actuator to hold the actuator and torsion spring in a preloaded condition. Upon brake release, the cocked torsion spring cooperates with the actuator and one-way clutch to drive the worm and advance the arm to a new adjusted position.
The present invention also comprehends the provision of a novel automatic slack adjuster for vehicle brakes utilizing a direct linkage between the brake actuating power source and the rotary actuator in the operating arm assembly; wherein the lower end of the link has an elongated slot formed therein receiving suitable means such as a pivot pin mounted in an arm of the actuator. The elongated slot provides a lost motion operation equal to the desired normal running clearance between the linings and the drum. Thus, the slack portion of an actuating stroke wherein the brake shoes are moved from a retracted position to a drum engaging position does not provide any rotation of the actuator or adjustment of the worm gear until there is a measurable amount of wear of the brake shoe linings.
Further objects are to provide a construction of maximum simplicity, efficiency, economy, and ease of assembly and operation, and such further objects, advantages, and capabilities as will later more fully appear and are inherently possessed thereby.